@Peter44Flynn
It’s probably HDR, rather than 4K as such, doing this.
But having made a note of your picture settings on HD material, go back to them on one of the offending 4Ks.
I found that my TV had a whole raft of different picture settings for UHD/HDR. Re-establishing my chosen settings there instead of the manufacturer’s defaults restored the proper lighter, and more vibrant, picture I preferred.
And these settings were retained for future excursions into UHD/HDR.
My favourite bedtime reading is a Sony product manual…
